In the past, many efforts have been expended to eliminate ripple (the fundamental frequency of load voltage) from control signals in closed-loop feedback control systems operating from an AC power source, since it is common that such ripple is (or could be) within the operating bandwidth of the control system and hence would interfere with performance.
One approach has been to limit operating bandwidth in order to avoid having such control systems respond to the ripple frequency. Another approach has been to use relatively complex (and hence costly) filters to eliminate such ripple.
However, certain errors will be present when various known ripple elimination techniques are used. For example, using a simple analog low-pass filter to eliminate ripple provides good steady state accuracy, but poor transient fidelity, and, when used on a feedback signal in a closed-loop control system, will tend to cause system instability.
Another technique which has been investigated is to perform synchronous sampling of a signal in a system having a fixed frequency or periodic ripple characteristic. This technique will reduce ripple, but is generally much less accurate than the simple first order low pass analog filter in steady state performance. For example, and by way of illustration, within a range where the signal is continuous, 10% accuracy can be achieved. Outside this range, however, where the signal is discontinuous, as is often the case, steady state error can approach 100% of the reading. Such errors are intolerable when the signal of interest is the prime variable to be controlled, as for example armature current in a motor operated from an AC power supply. In such a system, rapid transient response and accurate, tightly controlled steady state performance are often required.
In the digital domain, one technique which has been used to reject ripple is to sample a signal to be filtered at a rate typically ten or more times faster than the highest frequency component of interest in the signal, to avoid "aliasing" and distorting the signal. Unfortunately, this technique has been found to make prodigal use of digital systems' resources, tying-up hardware or operating overhead in signal capture and processing.
The present invention overcomes the shortcomings of the various approaches of the prior art by utilizing a combined analog and digital technique to provide rapid and high fidelity transient response and accurate steady state performance with a great reduction in digital system overhead in comparison to conventional 10x sampling techniques.